Chapter 12
Success rates of embryo transfer

MEASURES OF SUCCESS

For the cattle breeder, success should usually be measured in terms of marketable product or its equivalent, for example, increased net worth. However, factors such as inflation and interest rates complicate evaluations of net worth to such an extent that more broadly and internationally applicable measures of success are normally used, such as numbers of live calves at one month of age. This is a marketable product, and other measures of success can be derived from it.

For the provider of embryo transfer services, success rates are usually measured by the income received, which in turn generally depends on the number of transferable embryos recovered per donor and the pregnancy rate per recipient. Thus, success is measured at an earlier stage in the process than for the cattle breeder.

DETERMINANTS OF SUCCESS

Embryo transfer consists of a series of steps, all of which must be done well or failure will result. These steps are summarized in Table 9 along with comments concerning the degree of control that can be exercised over each step. As indicated in the right-hand column, the breeder or personnel providing the services that the breeder purchases have considerable control over embryo transfer success rates. The one factor that is nearly impossible to deal with is variability in the number of normal embryos produced in response to superovulation. This causes considerable frustration.

Several other items in this table deserve comment. One can never be sure of intrinsic fertility of donors, recipients, and semen, or of the viability of embryos, although one can certainly improve the chances of success by selecting donors and recipients from a population that has high fertility. It is probably not cost-effective to go to the lengths of testing a large batch of semen for fertility, or buying large batches of drugs for superovulation and using a batch shown to work well on hundreds of donors. Weather cannot be controlled directly, but one can avoid doing embryo transfer during seasons of the year when fertility is low.

Success rates of embryo transfer

TABLE 9
Steps in the embryo transfer process and the ability to deal with them successfully

TABLE 10
Factors that may alter success rates with embryo transfer

Some additional factors which can affect success rates with embryo transfer are listed in Table 10. Cows with histories of infertility generally produce only about one-third as many pregnancies as normal donors, although this depends on the kind of infertility (Bowen et al., 1978). Occasionally, results with infertile donors are satisfactory. Usually one has little choice in the second factor in Table 10, fresh versus frozen semen. Either kind of semen works well if handled properly, except that some bulls do not produce semen that freezes well.

An important point is that calves from embryo transfer are normal. We studied 1 900 embryo transfer pregnancies (King et al., 1985), and found no differences from non-embryo transfer calves in abortion rates, congenital abnormalities, birth weight, sex ratio (51 percent male), neonatal death, gestation length, calfhood disease or any other characteristic studied.

CURRENT PRODUCTION AVERAGES

On average, two to four calves will result per superovulated donor under the following field conditions: normal, fertile donors being superovulated for the first or second time, excellent management, well-trained embryo transfer personnel, sufficient synchronous recipients for the majority of embryos, surgical or non-surgical transfer by experienced technicians with proven skills, and unfrozen embryos. If embryos are frozen, the average will be 15–20 percent lower.

TABLE 11
Distribution of transferable embryos produced by superovulated donors

Table 11 contains approximate distributions of embryos recovered per donor superovulated derived from several sources (e.g. Looney, 1986). Note that no transferable embryos are recovered from 20–30 percent of donors and that about 80 percent of embryos are recovered from the 40 percent of donors that do best. Methodology has improved slightly since the data in Table 11 were collected, so current success rates could be slightly higher at embryo transfer units. Success rates would probably be lower on the farm.

Success rates can also be examined on the basis of pregnant recipients per donor, as is illustrated in Table 12 which gives the results of 64 consecutive superovulations of normal donors of beef breeds, taken from records at the Embryo Transfer Unit at Colorado State University. Data include all donors treated, even those that did not show oestrus, did not respond to superovulatory treatment or did not yield ova. Often animals with these problems are not included in averages.

Unfortunately, it is not possible to predict which donor will have one pregnancy and which will have ten or more. Note that 47 percent of the donors had zero, or only one or two pregnancies as a result of superovulation. If one superovulates each donor repeatedly, for instance, three times at two-month intervals, the variability in total pregnancies per donor evens out somewhat. With these 64 donors, an average of 3.4 pregnancies per donor per superovulation resulted. About 90 percent of recipients pregnant at three months of gestation will produce a live calf at one month of age with excellent management.

TABLE 12
Distribution of numbers of pregnancies from 64 superovulated donors